As such a proportional counter of this type, one as shown in FIG. 1 has so far been known. Namely, the same comprises a hollow cylindrical cathode (1) having end plates (3a), (3b) hermetically attached to both ends thereof, and an anode (2) mounted between suitable terminals (not shown) provided on the end plates, such that the anode is insulated from the cathode (1) and extends along the axis of the cathode (1). (4) is a gas-filling and evacuating pipe with its one end hermetically connected to the end plate (3a) to be used for filling BF.sub.3 gas (5) into the hermetic cylinder forming the cathode (1). The other end plate (3b) is provided with a connector (6) for withdrawing an electrical signal. The internal surface of the cathode (1) is coated with a variety of getters (7) having different operating temperatures, these getters (7) having absorptivity for fluorine gas.
Now the basic operation of a BF.sub.3 proportional counter will be described. When neutrons enter the BF.sub.3 proportional counter, they react on the isotope .sup.10 B in the BF.sub.3 gas filled in the counter and give rise to alpha particles and Li nuclei having high energy. These high energy charge particles ionize BF.sub.3 gas molecules, when these pass through the BF.sub.3 gas, thereby to give rise to electrons and BF positive ions. This phenomenon is called primary ionization. The thus produced electrons and positive ions are collected respectively by the anode (2) and the cathode (1) with a direct current voltage applied therebetween. In the course of the electrons being collected by the anode, because of a strong electric field in the vicinity of the anode, the electrons are greatly accelerated and, upon collision with BF.sub.3 gas molecules, ionize the BF.sub.3 gas molecules, producing new electron-ion pairs. The newly produced electrons likewise ionize the BF.sub.3 gas molecules. Thus, when these electrons are collected by the anode, the quantity of the charge becomes much larger than that produced in the beginning by the primary ionization. This process is called the gas amplification and normally the quantity of the charge produced by this process is evaluated by way of pulse counting, thereby to make neutron measurement.
Described above is the basic operation of the BF.sub.3 proportional counter.
The BF.sub.3 proportional counters of high sensitivity are readily available and are widely used for measurement of the low neutron flux. However, the durability thereof is not so good, and a disadvantage was involved that while only impingement of radiation without the counter being put in an operating condition merely entails little deterioration of the characteristic, placement of the counter in an operating condition even under the external radiation of not so high a level gives rise to deterioration in the characteristic within a relatively short period of time, whereby inconveniences are caused in the measurement of neutrons.
Not only the external radiation of neutrons but also that of gamma ray has the same effect in that the output pulses from the counter become gradually smaller.
To improve the radiation resistant characteristic of the counter, there is a report on a method in which, although little deterioration is caused if the counter is subjected only to radiation without the same being put in an operating condition, the counter is subjected to gamma ray radiation of at least 10.sup.6 Rads at a rate of at least 10.sup.5 Rad/hr while putting the counter in an operating condition, i.e. applying a high direct current voltage. This method results in unlikeliness of deterioration of the characteristic of the BF.sub.3 proportional counters after the same is subjected to the above described radiation of the gamma ray. However, the results of our detailed and extensive experiments revealed that disadvantages are involved that not only the method is not easy to practice since it requires a very strong gamma ray source for providing the high ray amount rate of 10.sup.5 Rad/hr or more but also subjecting of the counter to radiation of more than a necessary amount of gamma ray shortens the remaining life of the BF.sub.3 proportional counter after radiation.